Many types of logic, memory and display systems utilize arrays of bulk type or thin film semiconductor circuits having thousands of elements interconnected on individual wafers or substrates. As the number of circuit elements on a wafer or substrate interconnected to perform a logic, memory or display function becomes larger, the probability of a defective circuit element increases. As a result, the entire wafer may be rendered useless. If the number of flawed wafers is large, the wafer yield is low and implementation of the logic, memory or display function may be considered too costly.
One technique that has been found effective to improve wafer yield is the use of redundant circuit elements (i.e., the addition of circuit elements on the wafer that may be substituted for defective circuit elements). After wafer fabrication is completed, defective elements are located. The interconnections are then altered to replace the defective element with a redundant element. In many situations, however, the interconnections cannot be readily changed. It is therefore necessary to provide redundant sub-functions that include a large number of circuit elements and apparatus to automatically select the sub-function elements that are to be used.
U.S. Pat. No. 4,714,839 (issued Dec. 22, 1987 to Shine C. Chung) discloses a control circuit for disabling or enabling the provision of redundancy in a memory circuit in which a dynamic latch is coupled to the output of a high-low-high circuit. The high-low-high circuit activates redundant elements in response to fuse controlled complementary address signals and includes a first fuse to enable the redundant elements and a second fuse to disable the redundant elements. While the redundant elements are automatically substituted for the defective elements, the arrangement requires a plurality of fuses and addressing elements so that it is relatively complex and is applicable only to line substitution in memory type functions. It is desired to provide a less complex substitution arrangement that controls a smaller set of redundant elements.